176 



GENERAL PRINCIPLES OF ZOOLOGY. 



neously the constriction at one pole of the egg of the " di- 

 rective corpuscles " or " polar bodies." 



Formation of the Polar Bodies. The germinal vesi- 

 cle initiates the changes, its walls disappearing, its contents, 

 in part, mingling with the cytoplasm of the egg, in part, 

 probably being employed for the formation of a nuclear 

 spindle. The latter, called also the directive spindle, places 

 itself with its axis in a radius of the egg so that one pole is 

 turned towards the centre and the other fixed in the super- 

 ficial layer of the egg (Fig. 89, a). Now begins a regular 

 cell-division process, but the products of the division are 



FIG. 89. Successive stages in the formation of the polar bodies of Asterias glacialis. j/, 

 directive spindle; -, first polar body; r/i- 2 , second polar body; f, egg-nucleus In 

 process of formation. 



of very unequal size ; the larger part is the egg, the 

 smaller quite insignificant part is the polar body (Fig. 89, 

 b, c]. The latter raises itself as a projection above the 

 surface; into this the directive spindle pushes one half of 

 itself; in the cutting off this half of the spindle is included 

 in the polar body. 



The Second Polar Body. The part of the directive 

 spindle remaining in the egg is immediately perfected as 

 a new spindle; the cell-budding is repeated and leads to 

 the formation of the second polar body. As a result, two 

 small cells lie at one pole of the egg, in many cases even 

 three, since, during the formation of the second polar body, 



